Thin films for a variety of purposes for many years have been made by casting a layer of polymer and solvent onto a moving casting support member such as a drum or endless belt or band. The solvent evaporates from the layer while the layer moves through dryer zones on the casting support member, until the layer is sufficiently dry to be self supporting, whereupon the layer is removed from the casting support member. Casting is achieved in the familiar manner by forcing a dope of solvent and polymer through a narrow slot in a casting hopper or die which extends substantially across the width of the casting support member. As the dope flows through the slot, small globules or agglomerations sometimes tend to form along the lips adjacent the slot, due to tiny inclusions in the dope which stick to the lips, to irregularities in the ambient conditions at the lip and to complex combinations of such and other conditions. The presence of such globules or agglomerations can cause elongated irregularities to form in the cast layer as the layer moves from the slot to the casting support member. Such irregularities may not settle or dissipate as the cast layer dries, leading to a product film with surface irregularities which render it useless for various applications, such as the manufacture of photographic film.
One known technique for reducing such irregularities is to visually inspect the cast layer through windows in the film forming apparatus, before the cast layer leaves the casting support member or after the cast layer has been stripped from the casting support member, to detect the presence of any irregularities which may have been caused by such globules or agglomerations. The film forming apparatus is then stopped and its gas tight casing is opened to permit an operator to directly observe the lips of the casting hopper and wipe off any agglomerations using a solvent soaked swab, for example. Not only does this slow the production of the thin film, but also it exposes the operator to solvent vapors which may be harmful or explosive or both.
A need has existed for an apparatus and method for detecting and removing agglomerations from the lips of a casting die, without requiring that the foam forming apparatus be stopped or that the casing of the apparatus be opened with attendant release of solvent vapors.